A number of unique refrigeration machines have been developed to satisfy the increasing demand for cryogenic refrigeration. These devices, all based upon the controlled cycling of an expansible fluid with suitable heat exchange to obtain refrigeration, are exemplified by U.S. Pat. Nos. 3,333,433, 3,321,926, 3,625,015, 3,733,837, 3,884,259, 4,078,389 and 4,118,943, and the prior art cited in those patents.
The present invention is directed to an improved form of refrigeration system of the kind which employs a working volume defined in part by a displacer, with the latter arranged to reciprocate so as to cause an increase or decrease of the working volume, valve means for causing a refrigerant fluid to flow into or out of the working volume in accordance with movement of the displacer, and a thermal regenerator through which the refrigerant fluid flows to and from the working volume in accordance with movement of the displacer. Such systems may take various forms and employ various cycles, including the well known Gifford-McMahon, Taylor, Solvay and Split Stirling cycles. In each case the flow of refrigerant and the displacer movement must be controlled continuously and accurately so that the system can operate according to the particular refrigeration cycle for which the system is designed.
Heretofore, various means have been used to achieve the desired refrigeration cycle, including the use of different forms of valving and various means for achieving controlled movement of the displacer. Inevitably, the prior devices have suffered from engineering limitations. In certain cases, the valving employed to control the flow of refrigerant fluid has suffered from one or more of the following: undue complexity of construction, relatively high cost of manufacture, difficulty of adjustment as to operation, relatively short operating life, and suitability only for a small range of refrigeration capacities. Other problems which have plagued prior forms of cryogenic refrigerators having gas-driven displacers have been excessive size of the valving (or of the refrigerator because of the valving construction and/or location), the "slamming" or "banging" of the displacer each time it undergoes direction reversal, low efficiency due to distorted P.V. diagrams, excessive work input or work absorption (e.g. high friction losses), or inability to operate well at the low reciprocating speeds that are preferred for such apparatus. Another equally important limitation of some prior devices is the inability to accommodate, or to accommodate conveniently, dual stages of refrigeration. In this connection it is appreciated that it is old in the art to provide refrigerators with a first refrigeration stage capable of achieving a first selected low temperature, e.g. 77 degrees K, and a second stage connected in series with the first stage and adapted to achieve a second still lower temperature, e.g. 20 degrees K. However, the physical disposition of the heat exchangers which characterize the two stages may be such as to complicate the utilization of the refrigerator in selected applications, e.g. in a cryo pump. Furthermore, design limitations of prior refrigeration equipment may make it difficult or excessively expensive to modify the first or second stage cooling capacities for different applications.